5.1 Hair & Scalp Physiology

5.1 Hair & Scalp Physiology

Quick Answer: Hair is made of keratinized protein. The shaft has three layers — cuticle, cortex, and medulla — while the living root sits inside the follicle and is fed by the papilla. Hair's shape is held by hydrogen, salt, and disulfide bonds, and it grows in three phases: anagen, catagen, and telogen.

The Hair Shaft: Three Layers

The part of the hair you see above the scalp is the hair shaft, and it is technically dead — only the root contains living cells. Each shaft has up to three layers:

• Cuticle — the outermost layer, made of overlapping transparent scales like shingles on a roof. When the scales lie flat and tight, hair looks shiny and feels smooth; when they are raised, hair feels rough, tangles easily, and looks dull. Almost every service you perform (shampooing, coloring, chemical texture work) manipulates this layer.
• Cortex — the middle layer, making up about 90% of the hair's total weight. It contains long keratin chains and the melanin pigment that gives hair its color. All chemical changes — permanent waving, relaxing, haircoloring, lightening — happen in the cortex.
• Medulla — the innermost core, an open shaft of loose cells. It is not always present; fine and very light-colored hair often has no medulla at all.

Root Structures Below the Skin

The root of the hair lives inside a tube-like pocket in the skin:

• Hair follicle — the tube-like depression or pocket in the skin that surrounds the hair root.
• Hair bulb — the lowest part of the hair strand, a thickened club-shaped structure that fits over the papilla.
• Hair papilla — a small cone-shaped elevation at the base of the follicle containing the blood and nerve supply that provides nutrients for hair growth. This is the only structure whose destruction permanently stops hair growth.
• Sebaceous gland — an oil gland attached to the follicle that secretes sebum, an oily substance that naturally conditions the hair and skin.
• Arrector pili muscle — the tiny involuntary muscle attached to the follicle base; when it contracts (from cold or fear), it causes goose bumps and pushes sebum toward the skin surface.

Keratin and the Bonds That Shape Hair

Hair is about 90% protein, made of amino acids joined in long chains called polypeptide chains. The amino acids are connected by peptide bonds (also called end bonds). Cross-linking these chains are two kinds of side bonds:

• Weak side bonds — hydrogen bonds and salt bonds. These are broken by water or heat (wet setting, blow-drying, flat irons) and reform as the hair dries or cools. Hydrogen bonds account for about one-third of the hair's overall strength and salt bonds for about another third; together they are the physical basis of all temporary styling.
• Strong side bonds — disulfide bonds, which join two sulfur atoms in the amino acid cysteine. Water and heat cannot break them. Only chemical solutions (permanent wave solution, relaxers) break disulfide bonds; although there are far fewer of them than hydrogen or salt bonds, they are the strongest side bond and account for about one-third of the hair's overall strength. This is why chemical texture services are permanent — and why over-processing destroys hair.

The Growth Cycle: Anagen, Catagen, Telogen

Hair grows in repeating cycles, and each follicle cycles independently:

Scalp hair grows fastest between the ages of 15 and 30, and growth is generally faster in summer than in winter. Losing about 35 to 40 hairs per day is normal in a healthy adult; shedding beyond that range may signal alopecia or a scalp disorder.

Analyzing Texture, Density, Porosity, and Elasticity

Before any service, analyze the client's hair:

• Texture — the thickness or diameter of the individual strand: fine, medium, or coarse. Fine hair is fragile with little or no medulla; coarse hair is strong but can be harder to process.
• Density — the number of hairs per square inch of scalp (thin, medium, thick). Dense hair is not the same as coarse hair: a client can have coarse strands but low density.
• Porosity — the hair's ability to absorb moisture or liquid. Test by sliding fingers up a strand from end to scalp: a rough feel means raised cuticle (high porosity). Overly porous hair already absorbs chemicals too fast and processes unevenly, so it needs milder, lower-strength products and careful timing; stronger, more alkaline products are reserved for resistant (low-porosity) hair with a compact cuticle that resists penetration.
• Elasticity — the hair's ability to stretch and return without breaking. Normal dry hair stretches about 20%; wet hair can stretch up to 50% of its length and spring back. Low elasticity means weak, over-processed hair.

Gray Hair

Gray hair is caused by the loss of natural melanin pigment in the hair papilla as a person ages. An individual gray hair is actually white — it has simply lost its melanin. What we call "gray" hair is an optical illusion created by white hairs mingled with still-pigmented ones. Heredity is the main factor in when hair turns gray, though emotional strain and certain illnesses can play a role. Eumelanin provides dark (brown to black) pigment while pheomelanin provides red to yellow pigment; the mix of the two, plus the number and size of melanin granules, determines natural hair color.